Flux of topical pilocarpine to the human aqueous

Aqueous fluid was withdrawn from eyes of patients undergoing cataract extraction at various intervals after administration of two drops 2% pilocarpine-HCl in a standard manner. Determination of aqueous pilocarpine concentration was made both by spectroscopy of a ferric hydroxylamine complex and by gas-liquid chromatography. These methods were consistent in indicating that concentration does not rise beyond 5 micrograms/ml at any time following topical instillation. The mean of 71 GLC determinations of aqueous tapped between 2 and 32 minutes after drops was 1.67 micrograms/ml. With assumption of a total chamber volume of 400 microliter, the average total pilocarpine in aqueous in these circumstances is less than 1 microgram. These findings correlate well with investigations of transcorneal flux of pilocarpine for the rabbit in a partial in vitro transport chamber system, with which comparable low flux efficiency was found after simulated drop administration. This serves to validate in some measure in extrapolation of other findings in chamber experiments to the living human eye. The combined in vitro and in vivo experimental results suggest that two distinct mechanisms govern the flux of pilocarpine across the cornea. High doses, comparable to those in standard clinical use, whether administered in drops or in constant flow, are transported inefficiently with kinetics indicating a diffusional mechanism and are associated with intracorneal retention or degradation of a substantial moiety. Low doses, if continuously applied, are much more efficiently transported. Hydrogel polymer vehicles appear to mobilize this low-dose mechanism by retaining drug against mechanical dissipation and elution by tear flow, but also by retaining drug against the capability of the cornea to take up more pilocarpine than can be transported to produce an intracorneal drug "depot." Although the exact nature of the "depot" is not clear, it is not elutable as pharmacologically active drug. It is consistently associated with the relatively poor flux efficiency found with high doses, and thus may act in some manner to disable a more efficient mechanism. The flux efficiency found with hydrogel mediation is more than double the best found in constant flow determinations. Vehicular mediated flux is rate limited by the cornea, independent of dose, linear with time despite exponentially secreasing available drug, and not associated with an intracorneal drug "depot." These features are consistent with carrier mediation of some type.